1. Field of the Invention
The present invention relates to a process of producing hollow shaped articles such as fuel containers for storing fuels such as gasoline.
2. Description of the Prior Art
As automobile fuel tanks, metal fuel tanks produced by press working and welding of iron plates, etc. have been used if putting much weight on the mechanical strength, and blown synthetic resin fuel tanks have been used if the processability is important. Particularly, to reduce the weight of vehicles and broaden the design freedom, thermoplastic resin fuel tanks have recently come to be used.
In the production of large blown fuel tanks, the thickness of upper portion of parison becomes thinner than the thickness of its lower portion when a heated parison in a flowable state is inserted into a mold from the top of molding machine. As a result, the resultant shaped article lacks the uniformity of thickness in some cases. If the shape of fuel tank is quite irregular or complicated, the degree of expansion of parison in a mold is likely to be different from portion to portion, to make the thickness uneven. Therefore, it is difficult to control the thickness of products. To attain satisfactory performance in the mechanical strength, etc., the thickness of products should be increased throughout its entire portions. This, however, increases the weight of fuel tanks. In a blow molding, a parison is molded with its periphery cramped between the parting faces of mold. Therefore, relatively large-sized flashes are formed, increasing the amount of wasted materials and reducing the productivity.
Since the parison is expanded in a mold in a blow molding, it is limited to dispose a fuel pump unit, a float, etc. inside a fuel tank being produced. To remedy this drawback, it has been proposed to mold an upper part and a lower part separately and then bond these parts into an integral fuel tank (JP 10-157738A).
To protect the environment, fuel tanks are required to have fuel barrier properties so as to prevent fuel, etc. from escaping into surrounding atmosphere through its wall. To ensure this, fuel tanks have been conventionally produced in a manner in which a fuel barrier film is first vacuum-formed in a mold into a barrier layer having a shape of fuel tank, and then, a thermoplastic resin for a substrate layer is injection-molded on the barrier layer so as to allow the barrier layer to face the inside of fuel tanks. However, since the barrier layer is sometimes stretched during the injection molding, the fuel tanks thus produced may have insufficient barrier properties. Also, the adhesion or bond strength between the barrier layer and the substrate layer made of thermoplastic resin is insufficient in some cases. In addition, it becomes sometimes difficult to allow the barrier layer to fully and completely extend to the peripheral ends, to reduce the fuel barrier properties.
In JP 2001-129851A, a production process of two-part fuel tank is proposed, in which each of two parts is injection-molded in each mold, the molded parts are butted to each other by sliding the molds, and then, a molten resin is injected around the butting faces to fuse-bond the two parts into a united body. However, this process requires a complicated equipment for sliding the molds, to increase production costs.
JP 2004-98886A discloses a process of producing fuel tanks by integrally bonding two constitutional parts at their peripheries around open ends, in which each constitutional part is produced by the injection molding or injection press molding of a thermoplastic resin having a low fuel permeability.